ye olde chestnut

[Ancient Mariner]

intercooler air ducts face the wrong way? (My response to 4xoverland) | Auto Expert John Cadogan


No link as it does include a few poo words[bigsmile]

[101RRS]

Sorry not following - please explain.

[rick130]

StPierre-White or whatever his name is.
A mate sent it to me last night

Youtube is your friend

[Slunnie]

Good video, but I disagree with elements of both.

I agree with Cadogee in that the scoop is facing the right way for many of the reasons he outlined. Pressure differentials, temps etc with low pressure under the engine.

But 3nuts is correct in that there is a low pressure area at the base of the windscreen. What Cadogee hasn't realised is that he has modelled his aero off a car and not a 4WD, where the car is very aerodynamically designed to retain lamellar flow as they all do these days, but the Landcruiser is designed as a Landcruiser and the flow off the nose detaches as it lifts over the roof leaving turbulant low pressure air at the bottom of the windscreen and over the bonnet. What will exacerbate this is the back of the bonnet scoop where it drops again but the bonnet scoop helps to fill the low pressure area which improves the drag in that area. These things are not designed in the slightest like a modern aerodynamically efficient car.

Cabin air intakes don't sit in a high pressure zone, they're also always in that low pressure area which is dropped away in the well below bonnet height. If it was in the high pressure area it would pressurize the cabin and the air would come in a 110km/h. The reason why the air flows from the low pressure zone is because the cabin air exhausts from the rear of the cabin where there is a more siginificant low pressure zone from where the air detatches at the rear of the vehicle which creates a more sensible flow without ear problems.

Out of interest from testing and thesis investigations, a Series is more aero with a tyre on the bonnet because it fills the turbulent air area but if you did that to a Disco5 it would increase the drag. Also from LandRovers design, engineering and testing white paper, the new LandRovers have a Cd value very close to the 300ZX at 0.33.. but remember, it still has a larger area than the 300ZX

[rick130]

Slunnie, are you sure the base of the windscreen is a low pressure zone?

In all my reading it's a high pressure zone, and wool tuft testing a Defender (yes, you read that right) the flow was pretty laminar in the centre of the bonnet from front to near the back
However, the edges of the bonnet where it curves down to the guards and the tops of the guards was interesting, it was really disturbed and when I removed the blanking plate that is the fresh air intake/ventilation on LH drive cars the flow reversed, ie. air didn't come out, it went in.

Also, when lifting the rear of a GU bonnet with packers to see if it allowed air out to help with overheating it mase things worse, air was being pushed into the engine bay and wool tufts bore this out, it was a really strong high pressure zone at the base of the screen.

[Slunnie]

Slunnie, are you sure the base of the windscreen is a low pressure zone?

In all my reading it's a high pressure zone, and wool tuft testing a Defender (yes, you read that right) the flow was pretty laminar in the centre of the bonnet from front to near the back
However, the edges of the bonnet where it curves down to the guards and the tops of the guards was interesting, it was really disturbed and when I removed the blanking plate that is the fresh air intake/ventilation on LH drive cars the flow reversed, ie. air didn't come out, it went in.

Also, when lifting the rear of a GU bonnet with packers to see if it allowed air out to help with overheating it mase things worse, air was being pushed into the engine bay and wool tufts bore this out, it was a really strong high pressure zone at the base of the screen.

Yep, I'll just do a paintshop quality simulation. There isn't a lot of flow over the bonnet, where on a car there will be a lot of flow.

Potentially the GU sets up a low pressure area under the vehicle which creates that situation.

[Slunnie]

Slunnie, are you sure the base of the windscreen is a low pressure zone?

In all my reading it's a high pressure zone, and wool tuft testing a Defender (yes, you read that right) the flow was pretty laminar in the centre of the bonnet from front to near the back
However, the edges of the bonnet where it curves down to the guards and the tops of the guards was interesting, it was really disturbed and when I removed the blanking plate that is the fresh air intake/ventilation on LH drive cars the flow reversed, ie. air didn't come out, it went in.

Also, when lifting the rear of a GU bonnet with packers to see if it allowed air out to help with overheating it mase things worse, air was being pushed into the engine bay and wool tufts bore this out, it was a really strong high pressure zone at the base of the screen.

Sorry for the low quality, but you'll get the idea. Hopefully the pictures work here as I normally host them on my website.

For the top of the bonnet you can see that the airflow detatches off the front of the bonnet and then rejoins after the front of the roof. You can see on the pressure simulation that the pressure drops and on the speed simulation that the airspeed also drops. (Simulations 2, 3, 4).

For the GU, I'm really not sure, but you can see how the slightest of tweaks to the floor will change the pressure characteristic quite significantly which will have a massive effect on the way the air flows through the engine and why a top mount intercooler can work.... and why you probably don't want to drop a low hanging LPG or long range fuel tank into it. Compare simulations 1 and 3.

[JDNSW]

This reminds me of a discussion from nearly fifty years ago - the Citroen I drove at the time was considered to have much more realistic "real life" aerodynamic drag than any other car, simply because the self levelling suspension meant that attitude and ground clearance remained the same regardless of loading. Also helped that the bottom is smooth and aerodynamically designed, with a low pressure area for the airflow from the engine.

[rick130]

Sorry for the low quality, but you'll get the idea. Hopefully the pictures work here as I normally host them on my website.

For the top of the bonnet you can see that the airflow detatches off the front of the bonnet and then rejoins after the front of the roof. You can see on the pressure simulation that the pressure drops and on the speed simulation that the airspeed also drops. (Simulations 2, 3, 4).

For the GU, I'm really not sure, but you can see how the slightest of tweaks to the floor will change the pressure characteristic quite significantly which will have a massive effect on the way the air flows through the engine and why a top mount intercooler can work.... and why you probably don't want to drop a low hanging LPG or long range fuel tank into it. Compare simulations 1 and 3.Is this a type of CFD programme??!! [emoji50][emoji41]

[rick130]

And re the GU bonnet, previous experience with cars showed that would happen, as the under bonnet region is generally considered an area of high pressure so the pressure at the base of the screen would need to be higher still to push the will tufts back under the trailng edge.

I wish I'd taken video of it all.

One of the race car guys I worked with had an XJ6 Series II Jag at one point and he said the bonnet catch failed at 100km/h and it flew up to nearly the top of windscreen height.
Their bonnets are hinged at the front.

I wish I had a pressure differential gauge at the time I was playing with all that stuff just to really see what was going on.
Airflow isn't always what it seems

[JDNSW]

.....
Airflow isn't always what it seems

Yes. I am reminded of the Britten Norman Islander aircraft. Not long after it went into production the BN-2 was replaced by the BN-2a. The only difference was a changed profile of the rear of the engine housing, from duck tail to a wedge, and a fairing added to the undercarriage legs to increase the chord by about 30%.

The result was a marked increase in short field performance, despite an increase of 5% in both gross weight and load capacity (the modifications had no significant weight penalty). Just reduced turbulence round the engines and undercarriage legs.

[Slunnie]

Airflow isn't always what it seems

Thats exactly it. I smirked when Cadogee was into 3nuts about not knowing about the aerodynamics, but falling victim to his own accusations.

I think fluid dynamics is incredibly interesting but what people think they know really has to be tested via CFD or in a wind tunnel because subtle differences like windscreen rake (its a bit more than that), floors etc make such a big difference to flow, drag numbers, lift and aero balance.

[p38arover]

Point it out to him. [bigwhistle]

[Slunnie]

Point it out to him. [bigwhistle]

It'd be waste of my time I suspect.

To be honest Ron I see a fair bit of information that I know is wrong and I'm sick and tired of having the arguements with people that think they know, but haven't done the testing. So incorrect information then starts to get perpetuated, gains a critical mass and then becomes incorrect gospel. Incorrect information does my head in, and that other people then take that in good faith only to find out that its incorrect and it just cost them a lot money.... maybe $2500 on a set of tyres that scrub, and then probably don't even get the merest of apologies for their loss. There comes a point where you just have to say.... well I hope they fully research that before they cough up. Cadogee's and 3nuts discussion at the very least is inconsequential to virtually everybody and I have no doubt that he wouldn't hear it or reply to it anyway.

[DiscoMick]

Don't we need low pressure behind the radiator so air from the front is pulled through the radiator to increase cooling? If there was high pressure behind the radiator, such as from a forward-facing bonnet scoop, wouldn't hamper airflow through the radiator and engine temperatures would rise? Having vents in the mudguards back near the doors lets air escape and adds to the low pressure effect. Isn't that right?

[Slunnie]

Don't we need low pressure behind the radiator so air from the front is pulled through the radiator to increase cooling? If there was high pressure behind the radiator, such as from a forward-facing bonnet scoop, wouldn't hamper airflow through the radiator and engine temperatures would rise? Having vents in the mudguards back near the doors lets air escape and adds to the low pressure effect. Isn't that right?

If you check out the simulation 1 and 3 on the previous page you will see that there is pressure drop under the car in relation to on the nose which will move the air through. Gills really are just cosmetic and probably just vent pressure from the inner guard. Interestingly Toyota generate the same effect by the shape of the top rear of the guard.

https://www.superiorengineering.co/wp-content/uploads/2017/09/Toyota-Prado-120-Wagon-Silver-71357-1.jpg

[scarry]

Airflow isn't always what it seems

Tell me about it,i am sure you also have spent half your working life playing with all types of airflow......[bighmmm][biggrin]

[rick130]

Tell me about it,i am sure you also have spent half your working life playing with all types of airflow......[bighmmm][biggrin]Yep.

It was also really interesting to see what worked and didn't on race cars where aero stuff was pretty much banned.
Rad placement, boundary layers, side pods, strategic holes, all trying to make a car slipperier.

Even airflow going into the engine.
One dyno gain still has me a little stumped.
I was tasked with fabricating a new mounting so we could turn a K&N 90° to fit inside a new air box design that the engineer hoped would work better on the track. (Shaped a timber buck and then hand beaten aluminum base. It looked like a pressing when finished [emoji4])
Just turning the air cleaner picked up nearly 2HP.
We didn't believe it so removed it, fitted the old PiperX, (yuk by the way) fitted the K&N the way it was designed and even removed it.
The new orientation picked up power over no air cleaner. The standard K&N mount and no air cleaner are equal in HP/torque, the PiperX was worse than any other combination we tried.
It was changing the airflow as it entered the carby venturi.
Stunned but happy!

In those days only F1 and the big Sports Car builders had CFD

[Slunnie]

In those days only F1 and the big Sports Car builders had CFD

I'd never thought about that, if they use CFD with the engines, but it makes sense.